cmd: relocate cmd/go/internal/par to cmd/internal/par

cmd: relocate cmd/go/internal/par to cmd/internal/par

Relocate cmd/go's internal/par package up a level into
cmd/internal/par, so that it can be used by other cmd/internal
packages. No change in functionality. This change is intended to be in
support of making the cmd/go script test framework available to other
commands in addition to just the Go command.

Updates #68606.

Change-Id: I920a5d5c9b362584fabdbb2305414325b42856a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/601355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>